Temperature Compensation
The Temperature Compensation Window will allow you to set the temperature compensation method that will be used to apply small adjustment steps to the focuser as the temperature changes.
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None
This option will ignore TC allowing user to turn native focuser driver TC on and run independently
FocusMax
FocusMax will track the temperature using and apply focuser steps based on the temperature coefficient setting (steps / degree)
The Moving Average setting is the used to smooth out the readings in order reduce focuser movement from single momentary temperature readings
Min Move (steps) is the number of steps to be made when the temperature has changed
Enabling Apply Camera Idle will allow the focuser to move when the camera shutter is closed
Enabling Host Move Command will initiate a focuser move when a host application (CCDAutoPilot, ACP, etc.) has instructed FocusMax to move due to filter change.
Focuser driver
This option will allow the temperature compensation to be performed by the focuser driver. See your focuser manual for details in setting up the driver for this feature.
Camera Idle: Checking this option will instruct the focuser to move only when the camera shutter is closed to eliminate potential focuser shake during a focuser move event.
Host move command: Checking this option will instruct the focuser to move only when a host program (CCDAutoPilot, ACP, etc.) directs FocusMax to move the focuser such as when filter offsets.
It is common to see large out of focus donuts at the start of an observing session. Sometimes the donuts are so large that neither PinPoint or TheSkyX is able to plate solve a telescope position. This option will move the focuser to an estimated position based on the last successful autofocus run and temperature and current temperature measured by the focuser when FocusMax is opened..
Max temperature sets the max temperature difference between the temperature measured at the last successful autofocus and curent temperature measured by the focuser. If the delta exceeds this setting then the focuser is not automatically moved.
Display message (sec) is the amount of time that a message box will be shown which will allow you to perform abort the move. If the count down times out, the move will be made automatically. Note the information that is displayed in the count down message box.
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Temperature Compensation Coefficient: The temperature compensation coefficient has units of steps / degree. If the coefficient is known then it may be entered here if FocsuMax is performing the focuser temperature compensation or entered into the focuser driver.This box will be automatically updated if temperature compensation data is loaded into this window for analysis.
Moving Average: The moving average setting is the number of temperature points that will be collected and used to determine the average temperature. This will tend to smooth the data resulting in focuser movements when significant changes in temperature occur are found.
Minimum move:This setting will specify the number of focuser steps when a temperature compensation move is applied based on the temperature change.If you have a focuser with fine movement (such as FLI Atlas) then you may want to use a larger value
Pressing Open File will open a dialog window listing the temperature compensation files (XXX.TCL) that have been collected and saved (Preferences / General) using the Temperature Compensation Wizard.
The data will be automatically loaded into the data grid.
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The header section displays information from the temperature compensation file:
System Number
System Name
Focuser name
Date the temperature compensation data was collected
Temperature compensation file name
The data grid at the bottom of the window displays the actual data collected with the Temperature Compensation Wizard
Data point number
Use data point (Yes / No)
Focuser Position
Reported focuser Temperature
Date & Time
Julian Date
Analysis Window:
The analysis portion of the window displays:
Equation of the best fit line
Number of active data points
Sigma limits to be used for the analysis graph
Calculated Temperature Coefficient (slope from the best fit line)
Stand deviation of the data around the best fit line
Correlation coefficient (r)
The temperature coefficient are in units of steps/degree. If the slope is:
+ then the focuser position will decrease as the temperature drops
- then the focuser position will increase as the temperature drops
The linear correlation coefficient (r) measures the relative strength in the relationship between temperature and position in the data set.
The value of r will range between -1 and +1 (-1 <= r > =+1).
r = +1 or r = -1 is perfect correlation between temperature and position
r = 0 means there is no correlation is between temperature and position
r > 0.8 or r < -0.8 is generally considered a strong correlation
r = 0.5 is no better than a guess (flip of a coin)
The data set shown above was shows a very strong correlation of 0.9
Pressing the Graph button will generate a plot of the data with:
the best fit line
+ / - sigma lines specified on the Temperature compensation window (default 2.5) The sigma lines are used to help identify data points that look suspect. If a data point is beyond or close to the sigma limits then they can be turned off in the data grid by changing the 'Y' to 'N' by double clicking the row in the Use columns. This sill allow you to interactively examine the data points and note the change in the Standard Error box (smaller is better).
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1.Use the Temperature Compensation Wizard top collect temperature and focuser position throughout the night
2.Plan to dedicate several full nights to data collection at a time of year that you will experience the a large temperature swing from early evening throughout the night
3.It is highly recommended that you utilize AcquireStar to find a suitable star and automatically slew and center the star. If you do not use AcquireStar then you may want to select a star near the celestial pole to maximize observing time through the night and reduce tracking errors
4.Analyze your data using statistical methods above
5.Collect temperature compensation data over several nights but DO NOT combine the data into one dataset.
Compare the temperature coefficients from each night - do they have similar slopes?
Do the regression lines overlap or cross?
Are the lines offset from one night to the next but have the same slopes?
6.Not all focuser's are able to provide temperature readings that accurately track ambient temperature well.
The author has one focuser with the temperature probe installed in a box along with the electronics and was never able to find strong correlation between temperature and position.
Another focuser utilizes an external sensor that is placed near the primary mirror of the 16" Newtonian reflector.This focuser tracks ambient temperature with a strong correlation between temperature and position of 89%. Temperature compensation has allowed the author to reduce the autofocus frequency from 60 to 120 minutes.
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7.Some users may find that the temperature vs. position graph is not linear which may present a challenge for predicting focuser position as temperature changes.
